A development of an EL display using an electro luminescence (hereinafter, abbreviated as EL) element that is a light emitting element having features such as self-light emitting, a wide viewing angle, high-speed response, low voltage driving ability, and low power consumption has been in the spotlight.
An EL display is classified into a passive matrix type and an active matrix type according to a driving method of an EL element. In addition, it is classified into a top emission type and a bottom emission type according to a method of taking out emitted light. Here, it is referred to as a bottom emission type in case where emitted light is received from a glass substrate side on which an EL element is formed. Moreover, it is referred to as a top emission type in case where emitted light is received from the opposite side of a substrate on which an EL element is formed.
In an active matrix type EL display device, a thin film transistor (hereinafter abbreviated as TFT) is utilized for driving an EL element, and typically, after forming a driving circuit and the like comprising of a TFT on a substrate, an EL element is further formed over the driving circuit. Therefore, in an active matrix type EL display of a bottom emission type, since emitted light can not transmit through a region where a TFT or a wiring is formed, aperture ratio is limited according to the formation of a TFT or a wiring and thereby the amount of light which can be received is limited. On the other hand, in an active matrix type EL display of a top light emitting type, emitted light can be received without any constraints of aperture ratio according to the formation of a TFT or a wiring, and there is also no need to take into consideration of an effect on aperture ratio according to the formation of a TFT or a wiring. Therefore, designing a driving circuit and the like can have more flexibility.
In view of the aforementioned perspective, a development of an active matrix type EL display of a top light emitting type is advanced.
As a structure of an EL element used in an active matrix type EL display of a top light emitting type, a structure where an anode is formed on a substrate side (that is, a side where a TFT is formed) and a light emitting layer, a cathode, and the like are laminated in this order on the anode, and a structure where a cathode is formed on a substrate side and a light emitting layer, an anode, and the like are laminated in this order are conceivable.
As for an EL element, it is preferable to use an anode material in which work function is a conductive material. Therefore, as for an anode material, ITO (Indium Tin Oxide; work function is approximately 4.8 eV) is often used. However, in an EL element which has a structure where an anode is formed on a substrate side (that is, a side where a TFT is formed), then a light emitting layer, a cathode, and the like are laminated in this order, if a transparent conductive material such as ITO is used for an anode, emitted light transmits through the anode and leaks to a side where a TFT is formed. It is proposed to form an anode having a structure where a reflection film is formed under a film formed of ITO (an ITO film) in order to prevent light leak, and to further enhance lighting efficiency by reflecting light emitted to a lighting side.
As a top light emitting type EL display where an EL element having an anode in which an ITO film and a film comprising an opaque conductive material are laminated is used, the anode is formed on a planarizing insulating film on which an electrode (hereinafter, referred to as a source electrode) for transmitting an electric signal to a source (or a drain) of a TFT for driving an EL element is formed. Thereby there is a type where the electrode and the anode are connected through a contact hole (for example, refer to Japanese Patent Laid-Open No. 2000-77191 bulletin (Page 2–4, FIG. 1)).
However, in the aforementioned top light emitting type EL display, since a source electrode and an anode are formed of different layers, steps for forming an interlayer insulating film and a contact hole increase. Therefore, a development of a top light emitting type EL display having a structure that can shorten a step is required.
In view of the aforementioned problem, the object of the present invention is to provide a light emitting device of a top light emitting type integrated an electrode of light emitting element and a source electrode to reduce steps concerning a formation of an interlayer insulating film and a formation of a contact hole, and its manufacturing method.